You see, to fire the main cannon(the huge one), it has a mighty firepower. 5d6 i think.

But the UR...20!!!??? Who is gonna hit the target with that???

Really, i know i can fudge and stuff...but to fire this...20 UR??? Rayhawk, why did you made the rules like that? Is it supposed to be shot only by extremely lucky heroes??? For even luckier normal minifigs?!?!

I want to stick to the rules, but i don't know how to change it...

"You can get more of what you want with a kind word and a gun, that you can with just a kind word." - Al Capone
My official post number 1000 was "The whole battle?"

I say the best way to accomplish this is to subtract the target's size from the weapon's size and use the result to determine the UR.

Example: A size 3 mech with a size 3 giant gun can shoot another size 3 mech without penalty, at a UR of 3 as if he had a size 1 weapon. However, if he wanted to shoot a size 2 hardsuit the UR would be equivalent to a size 2 weapon. Firing at a size 1 minifig would have the UR of a size 3 weapon.

I've used an AT-AP in a battle. It didn't hit anything with the main gun. I think the main gun will only work vs very large vehicles or buildings. Another tactic you can try is to keep it stationary, and fire at stationary targets. Those together bump the Use down to 13.

You could also figure out something for that third leg, maybe using an action one turn and deploying it give another aiming bonus.

It's intentional that a weapon that large should have a Use rating that high. It limits the kinds of targets you can use it against. If you're firing against a large stationary fortification and you take time to aim, then you have a good chance to catch the target in the blast radius. If you're firing against a single enemy trooper, then he's almost impossible to hit and you're better off sticking with smaller weapons.

That being said, I did want to include some rules for targeting assistants, but I haven't figured out an elegant way to do it yet. The number of minifigs working together to fire a single gun would be limited to the Size number of the gun, but I'm not sure of the best way to stack their bonuses.